"Virtual" key word

can somebody explain to me what this keyword means?
i see it in alot of source, but i have no idea what the point behind it is:

virtual int GetSomeNumber(){ return m_Number;};

what is the diffrence of just using "int" and throwing a "virtual" infront of it?

A virtual function is a member function that you expect to be redefined in derived classes. When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived class's version of the function.

Code:

---

Example

class WageEmployee
{
public:
  virtual float computePay();
};

class SalesPerson : public WageEmployee
{
public:
  float computePay();
};

WageEmployee aWorker;
SalesPerson aSeller;
WageEmployee *wagePtr;

wagePtr = &aWorker;
wagePtr->computePay();  // call WageEmployee::computePay
wagePtr = &aSeller;
wagePtr->computePay();  // call SalesPerson::computePay

The virtual keyword is needed only in the base class's declaration of the function; any subsequent declarations in derived classes are virtual by default.

A derived class's version of a virtual function must have the same parameter list and return type as those of the base class. If these are different, the function is not considered a redefinition of the virtual function. A redefined virtual function cannot differ from the original only by return type.

---

You can also have pure virtual functions. They differ from virtual functions in that they must be redefined. This causes their class to become an abstract class. For example:

Code:

---

class Shape {
    virtual void Draw() = 0;
};

class Square : public Shape {
    void Draw() { }
};

class Circle : public Shape {
    void Draw() { }
};

---

This means that you cannot do this:

Code:

---

Shape x;

---

...or you'll get a "cannot instantiate abstract class" error from the compiler.

Pure virtual functions don't add any other significant functionality other than base class protection.

ok i get it now.. thanks alot =)